JP7491142B2 - Laser interferometer and method for controlling the laser interferometer - Google Patents

Description

The present invention relates to a laser interferometer and a method for controlling a laser interferometer.

Patent Document 1 discloses a laser vibrometer as a device for measuring the vibration velocity of an object, which irradiates the object with laser light and measures the vibration velocity based on the scattered laser light that has undergone a Doppler shift. This laser vibrometer uses optical heterodyne interferometry to extract the Doppler signal contained in the scattered laser light.

The laser vibrometer described in Patent Document 1 uses a piezoelectric element or a quartz crystal oscillator whose vibration frequency can be varied by changing the voltage, and the frequency is shifted by irradiating these vibration elements with laser light. By using the laser light containing the modulation signal whose frequency has been shifted in this way as a reference light, a Doppler signal is demodulated from the scattered laser light. The vibration velocity of an object can be measured using the Doppler signal obtained in this way.

Furthermore, Patent Document 1 describes that it is desirable to use a piezoelectric element as the vibration element, which has the property of deforming when voltage, magnetism, etc. are applied, and the vibration frequency can be varied by changing the voltage. It also describes that the vibration frequency must be a triangular wave or sawtooth wave whose waveform shows linearity at the rising edge, and that the optical Doppler shift caused by the incidence of laser light is utilized when the sawtooth wave applied voltage rises, or when the triangular wave applied voltage rises and falls.

JP 2007-285898 A

However, piezoelectric elements that are generally driven at frequencies above the kHz band and vibration elements such as quartz crystal resonators with high Q values use simple harmonic drive. For this reason, the driving method using triangular waves or sawtooth waves as described in Patent Document 1 does not provide sufficient modulation signal accuracy and is therefore not practical. Therefore, it is necessary to use a driving method that does not use triangular waves or sawtooth waves.

On the other hand, when using a drive method using a sine wave, which is relatively easy to achieve high accuracy, the speed of the part of the vibration element that modulates the laser light is likely to change from moment to moment. This causes the modulation frequency to change accordingly. If the frequency of the modulation signal changes in this way, a problem arises in that when demodulating a sample signal such as a Doppler signal from the scattered laser light, the sample signal cannot be accurately demodulated.

A laser interferometer according to an application example of the present invention includes:
A light source unit that emits a first laser beam;
an optical modulator including a vibration element, the optical modulator modulating the first laser light by using the vibration element to generate a second laser light including a modulation signal;
a light receiving element that receives interference light between a third laser light including a sample signal generated by reflection of the first laser light from an object and the second laser light, and outputs a light receiving signal;
a demodulation circuit that performs a demodulation process to demodulate the sample signal from the received light signal;
Equipped with
The demodulation circuit is characterized in that it performs the demodulation process intermittently.

A method for controlling a laser interferometer according to an application example of the present invention includes the steps of:
A light source unit that emits a first laser beam;
an optical modulator including a vibration element, the optical modulator modulating the first laser light by using the vibration element to generate a second laser light including a modulation signal;
a light receiving element that receives interference light between a third laser light including a sample signal generated by reflection of the first laser light from an object and the second laser light, and outputs a light receiving signal;
A method for controlling a laser interferometer comprising:
The optical fiber is characterized in that a demodulation process for demodulating the sample signal from the received light signal is performed intermittently.

1 is a schematic configuration diagram showing a laser interferometer according to a first embodiment. 2 is a conceptual diagram showing a first configuration example of the optical modulator shown in FIG. 1 . 1. FIG. 4 is a conceptual diagram showing a second configuration example of the optical modulator shown in FIG. FIG. 2 is a conceptual diagram showing a third configuration example of the optical modulator shown in FIG. 1 . 11 is a graph showing the relationship between the voltage applied to a drive section of a MEMS vibrating mirror element and the measured values of the maximum vibration velocity, maximum modulation frequency, and displacement amplitude of the MEMS vibrating mirror element. FIG. 13 is a conceptual diagram showing a fourth configuration example of the optical modulator shown in FIG. FIG. 2 is a block diagram showing a demodulation circuit that performs demodulation processing using a quadrature detection method. 1 is a graph showing a change over time in the position of a light reflecting surface provided on a vibration element that vibrates simply, and a graph showing a change over time in the modulation frequency caused by this vibration element. 13 is a graph showing the change over time in Doppler shift output from a demodulation circuit when a laser beam is irradiated onto an object to be measured moving at a constant speed. 10 is a table showing the relationship between the wave number corresponding to the time when the modulation signal contained in the reference light modulated by the vibration element is in the region of error of 1% or less shown in FIG. 9 and the maximum modulation frequency of the vibration element. FIG. 11 is a schematic configuration diagram showing an example of a mounting structure of an optical system included in a laser interferometer according to a second embodiment. FIG. 11 is a schematic configuration diagram showing an example of a mounting structure of an optical system included in a laser interferometer according to a second embodiment. FIG. 11 is a schematic configuration diagram showing an example of a mounting structure of an optical system included in a laser interferometer according to a second embodiment.

The laser interferometer and the method for controlling the laser interferometer of the present invention will be described in detail below with reference to the embodiments shown in the attached drawings.

1. First Embodiment First, a laser interferometer according to a first embodiment will be described.
FIG. 1 is a schematic diagram showing the configuration of a laser interferometer according to the first embodiment.

The laser interferometer 1 shown in FIG. 1 has an optical system 50, a demodulation circuit 52 to which a signal from the optical system 50 is input, and a control circuit 53.

The optical system 50 includes a light source unit 2, a polarizing beam splitter 4, a quarter-wave plate 6, a quarter-wave plate 8, an analyzer 9, a light receiving element 10, a frequency shifter-type optical modulator 12, and a set unit 16 in which an object to be measured 14 is disposed.

The light source unit 2 emits an emission light L1 (first laser light) of a predetermined wavelength. The light receiving element 10 converts the received light into an electrical signal. The optical modulator 12 includes a vibration element 3A, and modulates the emission light L1 to generate a reference light L2 (second laser light) containing a modulated signal. The setting unit 16 is adapted to allow the object 14 to be placed. The emission light L1 incident on the object 14 is reflected as an object light L3 (third laser light) containing a sample signal, such as a frequency signal or a phase signal.

The optical path of the emitted light L1 emitted from the light source unit 2 is referred to as optical path 18. Furthermore, optical path 18 is coupled to optical path 20 by reflection from the polarizing beam splitter 4. On optical path 20, a quarter-wave plate 8 and an optical modulator 12 are arranged in this order from the polarizing beam splitter 4 side. Furthermore, optical path 18 is coupled to optical path 22 by transmission through the polarizing beam splitter 4. On optical path 22, a quarter-wave plate 6 and a set unit 16 are arranged in this order from the polarizing beam splitter 4 side.

In addition, the optical path 20 is coupled to the optical path 24 by transmission through the polarizing beam splitter 4. On the optical path 24, an analyzer 9 and a light receiving element 10 are arranged in this order from the polarizing beam splitter 4 side.

The outgoing light L1 emitted from the light source unit 2 passes through the optical path 18 and the optical path 20 and is incident on the optical modulator 12. The outgoing light L1 also passes through the optical path 18 and the optical path 22 and is incident on the object under test 14. The reference light L2 generated by the optical modulator 12 passes through the optical path 20 and the optical path 24 and is incident on the light receiving element 10. The object light L3 generated by reflection at the object under test 14 passes through the optical path 22 and the optical path 24 and is incident on the light receiving element 10.
Each part of the laser interferometer 1 will now be described in order.

1.1.1. Light Source Unit The light source unit 2 is a laser light source that emits coherent emission light L1 with a narrow linewidth. When the linewidth is expressed as a frequency difference, a laser light source with a linewidth in the MHz range or less is preferably used. Specific examples include gas lasers such as HeNe lasers, DFB-LDs (Distributed Feedback - Laser Diodes), FBG-LDs (Laser Diodes with Fiber Bragg Gratings), and semiconductor lasers such as VCSELs (Vertical Cavity Surface Emitting Lasers). Of these, semiconductor lasers enable the light source unit 2 to be made smaller.

1.1.2. Polarizing beam splitter The polarizing beam splitter 4 is an optical element that splits incident light into transmitted light and reflected light. The polarizing beam splitter 4 also has the function of transmitting P-polarized light and reflecting S-polarized light, and can separate the polarization state of the incident light into orthogonal components. Hereinafter, we will consider the case where output light L1, which is linearly polarized and has a ratio of P-polarized light and S-polarized light of, for example, 50:50, is incident on the polarizing beam splitter 4.

As described above, the polarizing beam splitter 4 reflects the S-polarized light of the output light L1 and transmits the P-polarized light.

The S-polarized light of the output light L1 reflected by the polarizing beam splitter 4 is converted to circularly polarized light by the quarter-wave plate 8 and enters the optical modulator 12. The circularly polarized light of the output light L1 that enters the optical modulator 12 is frequency shifted by f _M (Hz) and reflected as reference light L2. Therefore, the reference light L2 contains a modulation signal with a modulation frequency f _M (Hz). The reference light L2 is converted to P-polarized light when it passes through the quarter-wave plate 8 again. The P-polarized light of the reference light L2 passes through the polarizing beam splitter 4 and the analyzer 9 and enters the light-receiving element 10.

The P-polarized light of the output light L1 that has passed through the polarizing beam splitter 4 is converted to circularly polarized light by the quarter-wave plate 6 and enters the moving object 14 under test. The circularly polarized light of the output light L1 that has entered the object 14 under test is subjected to a Doppler shift of f _d (Hz) and is reflected as object light L3. Therefore, the object light L3 contains a frequency signal with a frequency of f _d (Hz). The object light L3 is converted to S-polarized light when it passes through the quarter-wave plate 6 again. The S-polarized light of the object light L3 is reflected by the polarizing beam splitter 4, passes through the analyzer 9 and enters the light-receiving element 10.

As mentioned above, the emitted light L1 is coherent, so the reference light L2 and the object light L3 are incident on the light receiving element 10 as interference light.

In addition, a non-polarized beam splitter may be used instead of the polarized beam splitter. In this case, the quarter-wave plate 6 and the quarter-wave plate 8 are not required, and the number of parts can be reduced, thereby making the laser interferometer 1 more compact.

1.1.3. Analyzer The analyzer 9 is used because the mutually orthogonal S-polarized and P-polarized light are independent of each other, and so simply superimposing them does not produce interference. Therefore, the light waves in which S-polarized and P-polarized light are superimposed are passed through an analyzer 9 that is tilted at 45 degrees with respect to both the S-polarized and P-polarized light. By using the analyzer 9, it is possible to transmit light of components that are common to each other, causing interference. As a result, the analyzer 9 generates interference light with a frequency of f _M -f _d (Hz).

1.1.4. Light-receiving element The reference light L2 and the object light L3 are incident on the light-receiving element 10 via the polarizing beam splitter 4 and the analyzer 9. As a result, the reference light L2 and the object light L3 undergo optical heterodyne interference, and interference light having a frequency of f _M -f _d (Hz) is incident on the light-receiving element 10. By demodulating a sample signal such as a frequency signal or a phase signal from this interference light using a method described below, it is possible to finally determine the movement of the object 14 to be measured, that is, the speed, vibration, or displacement. The light-receiving element 10 may be, for example, a photodiode.

1.1.5. Optical Modulator Hereinafter, the optical modulator 12 will be described with four configuration examples based on the form of the vibration element.

1.1.5.1 First Configuration Example First, a description will be given of a first configuration example of the optical modulator 12. Fig. 2 is a conceptual diagram showing the first configuration example of the optical modulator 12 shown in Fig. 1.

The optical modulator 12 shown in FIG. 2 has a vibration element 3A that includes a piezoelectric element. A piezoelectric element has the property of deforming when a voltage is applied, and the vibration frequency can be changed by changing the voltage. The vibration element 3A has an element body 31 that is a piezoelectric element, and a light reflecting film 32 provided on the element body 31.

By having such a piezoelectric element in the vibration element 3A, it is possible to realize an optical modulator 12 that has a simple structure and is easy to reduce cost.

The element body 31 shown in FIG. 2 has a vibration mode in which it vibrates in an expanding and contracting manner in the vertical direction of FIG. 2. The element body 31 is made of a piezoelectric material. Examples of the piezoelectric material include piezoelectric ceramics such as lead zirconate titanate (PZT), barium titanate, and lead titanate, and piezoelectric plastics such as polyvinylidene fluoride.

The light reflecting film 32 is composed of a mirror film that spreads so as to intersect with the vibration direction of the element body 31. Examples of the mirror film include a metal film and a dielectric multilayer film.

As described above, such a vibration element 3A generates a simple harmonic motion in the vertical direction in FIG. 2. Simple harmonic motion refers to a reciprocating motion with a predetermined amplitude and a predetermined period. The amplitude and period of the simple harmonic motion can be controlled with relatively high precision by making the waveform of the voltage signal applied to the vibration element 3A a sine wave or a waveform similar to that. Therefore, by using a vibration element 3A that generates a simple harmonic motion, the vibration speed of the light reflecting surface can be controlled with high precision. This makes it possible to generate a highly accurate and stable modulation signal.

In the vibration element 3A, the upper surface of the light reflecting film 32 becomes the light reflecting surface that reflects the emitted light L1. The vibration element 3A may be an element that vibrates in-plane, i.e., vibrates parallel to the light reflecting surface, but in this embodiment, it is an element that vibrates out-of-plane, i.e., vibrates in an out-of-plane direction that intersects with the light reflecting surface. By using such a vibration element 3A that vibrates in an out-of-plane direction, the advantage is obtained that it is easier to miniaturize the optical modulator 12.

1.1.5.2. Principle of Frequency Shift by Optical Modulator Next, the principle of frequency shift by the optical modulator 12 will be explained. Prior to that explanation, however, the principle of Doppler shift by the object under test 14 will first be explained.

When the output light L1 with frequency _f0 is irradiated onto the object under test 14 moving with the V vector, the object under test 14 undergoes a frequency shift due to the Doppler effect. If the frequency shift (Doppler shift) due to the object under test 14 is _fd , then object light L3 with frequency _f0 + _fd is scattered from the object under test 14. The Doppler shift _fd can be calculated by the following formula (1).

Here, consider a case where the object 14 to be measured is moving at a speed V in a direction opposite to the incident direction of the outgoing light L1, and the incident outgoing light L1 is reflected by the object 14 to trace the same optical path as the incident light in the opposite direction. In this case, k _s = -k _i , so the above formula (1) becomes the following formula (2).

As is clear from the above formula (2), the Doppler shift _fd is derived in a linear relationship with the velocity V of the object 14. Therefore, if the Doppler shift _fd can be obtained by the laser interferometer 1, it becomes possible to measure the velocity V of the object 14 in a non-contact manner and without performing a calibration procedure.

Next, the principle of frequency shift by the optical modulator 12 will be described.
When the output light L1 is incident on the light reflecting film 32 of the optical modulator 12, the output light L1 is frequency modulated by the Doppler effect, and a reference light L2 containing a modulated signal is generated. If the frequency of the modulated signal is a modulation frequency _fM , the modulation frequency _fM can be calculated by the following formula (3).

The k _i vector is the wave vector of the output light L1 entering the optical modulator 12, and the k _s vector is the wave vector of the scattered light scattered by the optical modulator 12. Since the scattered light can be regarded as reflected light in the optical reflecting film 32, the k _s vector can be regarded as the wave vector of the reflected light. Furthermore, the v vector is the speed of the optical reflecting surface.

Here, consider that the outgoing light L1 is perpendicularly incident on the light reflecting surface. In this case, k _s =-k _i , and therefore the above formula (3) becomes the following formula (4).

θ is the angle between the direction of travel of the reference light L2 emitted from the optical modulator 12 and the speed direction of the light reflecting surface. Also, λ is the wavelength of the emitted light L1 that is incident on the light reflecting surface.

Now consider the case where the traveling direction of the reference light L2 and the speed direction of the light reflecting surface are the same. In this case, θ = 0, so the above formula (4) becomes the following formula (5).

On the other hand, consider the speed of the light reflecting surface. The displacement amplitude of the light reflecting surface is L ₀ (m), and the vibration frequency of the light reflecting surface is f _a (Hz). In this case, the position L (m) of the light reflecting surface is calculated by the following formula (6), and the speed v (m/s) of the light reflecting surface is calculated by the following formula (7).

From these equations (6) and (7), the above-mentioned modulation frequency _fM can be obtained by the following equation (8).

As can be seen from the above formula (8), the modulation frequency _fM varies in accordance with the simple harmonic motion of the vibration element 3A. The instantaneous maximum modulation frequency _fMmax is calculated by the following formula (9).

Here, consider a case where the traveling direction of the reference light L2 coincides with the maximum speed direction of the light reflecting surface. In this case, since θ=0 in equation (9), the maximum modulation frequency _fMmax can be obtained by the following equation (10).

As described above, in order to increase the maximum modulation frequency _fMmax , it is preferable that θ = 0 or a value close to 0. In light of this, it is preferable that the vibration element 3A is an element that vibrates out of plane rather than an element that vibrates in plane.

The vibration element 3A can change the displacement amplitude _L0 and the vibration frequency f _a by changing the applied voltage and frequency, thereby making it possible to adjust the maximum modulation frequency f _Mmax .

1.1.5.3 Second Configuration Example Next, a description will be given of a second configuration example of the optical modulator 12. Fig. 3 is a conceptual diagram showing the second configuration example of the optical modulator 12 shown in Fig. 1.

The second configuration example will be described below, but in the following explanation, the differences from the first configuration example will be mainly described, and explanations of similar points will be omitted. Note that in FIG. 3, the same reference numerals are used for configurations similar to the above configuration example.

The optical modulator 12 shown in FIG. 3 includes a vibration element 3A including a piezoelectric element and a mirror 33.

3, the light of the k _i vector incident on the light reflecting surface of the vibration element 3A is reflected by the light reflecting surface and enters the mirror 33 as light of the k _1s vector. Since the light of the k _1s vector enters the mirror 33 at an incident angle of zero, the exit angle is also zero, and the light of the k 1s vector enters the light reflecting surface of the vibration element 3A at the same angle. The light of the k _1s vector is reflected again by the light reflecting surface and travels along the same optical path as the light of the k _i vector.

By passing the light through the mirror 33 in this way, the reference light L2 generated by the optical modulator 12 is subjected to two frequency modulations. Therefore, by using the mirror 33 in combination, higher frequency modulation is possible compared to using the vibration element 3A alone.

Of the two reflections by the light reflecting surface described above, the modulation frequency due to the first reflection is designated as f _M1 , and the modulation frequency due to the second reflection is designated as f _M2 .

f ₀ is the frequency of the output light L1, and since there is a relationship of f ₀ >>f _M , the modulation frequencies f _M1 and f _M2 can be considered to be equal to f _M , as shown in the following equations (11) and (12), respectively.

In light of this, although the reference light L2 generated by the optical modulator 12 shown in Fig. 3 is the output light L1 given a frequency shift of the modulation frequency _fM ', this modulation frequency _fM ' can be regarded as _2fM . In that case, _fM ' is given by the following equation (13).

As a result, it can be seen that this configuration example enables higher frequency modulation. The second configuration example described above also provides the same effect as the first configuration example.

1.1.5.4 Third Configuration Example Next, a description will be given of a third configuration example of the optical modulator 12. Fig. 4 is a conceptual diagram showing the third configuration example of the optical modulator 12 shown in Fig. 1.

The third configuration example will be described below. In the following explanation, the differences from the first configuration example will be mainly described, and the explanation of the similar points will be omitted. In addition, in FIG. 4, the same reference numerals are used for the same configuration as the previous configuration example.

The optical modulator 12 shown in FIG. 4 has a vibration element 3B including a MEMS vibration mirror element. MEMS (Micro Electro Mechanical Systems) stands for microelectromechanical systems. The MEMS vibration mirror element has a movable part 34 suspended between torsion bars (not shown) and a light reflecting film 32 provided on the surface of the movable part 34.

The movable part 34 is formed together with the torsion bar using MEMS technology. The movable part 34 rotates back and forth around the rotation axis Ax, with the torsion bar as the rotation axis Ax. Methods for driving the movable part 34 include, for example, an electromagnetic drive method, an electrostatic drive method, a piezoelectric drive method, etc.

On the other hand, since the movable part 34 performs a reciprocating rotational motion, the light reflecting surface faces various directions with respect to the optical axis of the incident light. Therefore, the light of the k _i vector (emitted light L1) incident on the light reflecting surface is scattered in various directions according to the orientation of the light reflecting surface. In FIG. 4, as examples of light reflected in various directions by the light reflecting surface, light of the k _1s vector, light of the k _2s vector, and light of the k _3s vector are illustrated. All of these correspond to the reference light L2.

Here, in the demodulation process described later, the execution period of the intermittent process is set to the time zone where the vibration speed of the vibration element 3B is the largest, thereby improving the demodulation accuracy of the sample signal. Therefore, in the case of the vibration element 3B, when the light reflecting surface is located at the center of the rotation angle width, the vibration speed is the largest. When the vibration speed of the light reflecting surface is maximized in this way, the light that is the light of the k _i vector, which is the incident light, is reflected, and the light path overlaps with the light of the k _i vector, which is the incident light, is the light of the k _1s vector. If the light of this k _1s vector can be selectively extracted, other light is less likely to be mixed in, so that the signal-to-noise ratio (S/N) of the sample signal can be increased in the demodulation process.

The optical modulator 12 shown in FIG. 4 includes a light shielding section 35. The light shielding section 35 has an opening 351. The light shielding section 35 is configured to pass light of the k _1s vector through the opening 351 and to shield other light. This makes it possible to suppress light having vectors other than the k _1s vector, such as light of the k _2s vector and light of the k _3s vector, from entering the light receiving element 10. As a result, components that reduce the S/N ratio of the sample signal are cut by the light shielding section 35, so that the sample signal can be demodulated more accurately.

The light-shielding portion 35 is not limited in terms of its constituent material, etc., so long as it is a material that has a light-shielding function. In addition, instead of a material that has a light-shielding function, a material that ultimately blocks light through refraction, reflection, scattering, etc. may be used.

As described above, in this configuration example, the vibration element 3B has a MEMS vibrating mirror element. With this configuration, the characteristics of the MEMS vibrating mirror element can be effectively utilized. In other words, it is possible to realize a vibration element 3B that is easy to miniaturize, has a sufficiently large drive frequency, and has a large displacement amplitude. As a result, it is possible to sufficiently widen the frequency band or speed range of the measurable movement of the object to be measured 14, and a small, high-performance laser interferometer 1 can be realized.

In this configuration example, the optical modulator 12 is provided on the optical path of the reference light L2 (second laser light) and includes a light blocking section 35 that blocks a part of the reference light L2. In other words, the optical modulator 12 shown in Fig. 4 includes a light blocking section 35 that transmits the light of the k _1s vector shown in Fig. 4 and blocks the light of the k _2s vector and the light of the k _3s vector.

According to this configuration, the light of the k _1s vector reflected by the light reflecting surface during the time period when the vibration velocity of the vibration element 3B is the largest can be selectively guided to the light receiving element 10. This can improve the demodulation accuracy of the sample signal.

In addition, the MEMS vibrating mirror element can also change the displacement amplitude _L0 and the vibration frequency f _a by changing the voltage and frequency applied to the driving part that drives the movable part 34. This makes it possible to adjust the maximum modulation frequency f _Mmax .

Figure 5 is a graph showing the relationship between the voltage applied to the drive unit of the MEMS vibrating mirror element and the measured values of the maximum vibration speed, maximum modulation frequency, and displacement amplitude of this MEMS vibrating mirror element. Note that Figure 5 shows the measurement results when the vibration frequency of the MEMS vibrating mirror element is set to 1235 Hz and it is driven at the resonant frequency. The graph showing the maximum modulation frequency also shows the measurement results when light with a wavelength of 632 nm and light with a wavelength of 850 nm are used as the light incident on the MEMS vibrating mirror element.

In Fig. 5, it can be seen that when the voltage applied to the driving part of the MEMS vibrating mirror element is increased, the measured values of the maximum vibration speed, the maximum modulation frequency, and the displacement amplitude all increase. Based on this result, it can be seen that the maximum vibration speed, the maximum modulation frequency, and the displacement amplitude can be adjusted by changing the driving conditions.
In the third configuration example as described above, the same effects as in the first configuration example can be obtained.

1.1.5.5 Fourth Configuration Example Next, a description will be given of a fourth configuration example of the optical modulator 12. Fig. 6 is a conceptual diagram showing the fourth configuration example of the optical modulator 12 shown in Fig. 1.

The fourth configuration example will be described below. In the following explanation, the differences from the third configuration example will be mainly described, and the explanation of the similar points will be omitted. In addition, in FIG. 6, the same reference numerals are used for the same configuration as the previous configuration example.

The optical modulator 12 shown in FIG. 6 includes a vibration element 3C having a vibration arm 36 made of a silicon vibrator or a quartz crystal vibrator. As shown in FIG. 6, the vibration arm 36 is a so-called cantilever type vibration arm that is supported at one end and arranged so that the other end is a free end. In addition, the vibration arm 36 is provided with a light reflecting film 32.

The vibration arm 36, which is made of a silicon vibrator, is driven by a drive unit (not shown) and vibrates in a bending manner in the thickness direction. Examples of the drive system of the drive unit include an electromagnetic drive system, an electrostatic drive system, and a piezoelectric drive system.

On the other hand, the vibrating piece 36, which is made of a quartz crystal, vibrates in a bending manner in the thickness direction due to the inverse piezoelectric effect exhibited by the quartz crystal.

Since such a vibrating arm 36 performs a bending reciprocating motion, the light reflecting surface faces various directions with respect to the optical axis of the incident light, and therefore the light of the k _i vector (output light L1) that is incident on the light reflecting surface is scattered in various directions according to the orientation of the light reflecting surface. In Fig. 6, light of the k _1s vector, light of the k _2s vector, and light of the k _3s vector are illustrated as examples of light reflected in various directions by the light reflecting surface. All of these correspond to the reference light L2.

The optical modulator 12 shown in FIG. 6 includes a light shielding section 35. The light shielding section 35 has an opening 351. The light shielding section 35 is configured to pass light of the k _1s vector through the opening 351 and to shield other light. This makes it possible to suppress light having vectors other than the k _1s vector, such as light of the k _2s vector and light of the k _3s vector, from being incident on the light receiving element 10. As a result, components that reduce the S/N ratio of the sample signal are cut by the light shielding section 35, so that the sample signal can be demodulated more accurately.

In addition, the silicon vibrator and the quartz crystal vibrator can also change the displacement amplitude _L0 and the vibration frequency f _a by changing the voltage and frequency applied to the driving unit and the electrodes. This makes it possible to adjust the maximum modulation frequency f _Mmax .

As described above, in this configuration example, the vibration element 3C has a silicon vibrator or a quartz crystal vibrator. With this configuration, the characteristics of the silicon vibrator or the quartz crystal vibrator can be effectively utilized. In other words, it is possible to realize a vibration element 3C that is easy to miniaturize, has a sufficiently large driving frequency, and has a large displacement amplitude. As a result, it is possible to sufficiently widen the frequency band or speed range of the measurable movement of the measured object 14, and to realize a small-sized, high-performance laser interferometer 1.
In the fourth configuration example as described above, the same effects as in the first configuration example can be obtained.

1.2 Demodulation Circuit The demodulation circuit 52 performs demodulation processing to demodulate the light receiving signal output from the light receiving element 10 into a sample signal such as a frequency signal or a phase signal. There is no particular limitation on the method for demodulating the sample signal, but a well-known quadrature detection method is an example. The quadrature detection method is a method of performing demodulation processing on the light receiving signal by performing an operation of externally mixing mutually orthogonal signals on the light receiving signal.

Figure 7 is a block diagram showing a demodulation circuit that performs demodulation processing using quadrature detection. The configuration of the demodulation circuit shown in Figure 7 is a circuit configuration of a known digital circuit, and is used to demodulate a frequency signal from a received light signal based on light modulated using an optical modulator such as an acousto-optical element (AOM) whose modulation frequency does not change.

The demodulation circuit 52 shown in FIG. 7 includes a high-pass filter 521, multipliers 522a and 522b, local oscillators 523a and 523b, low-pass filters 525a and 525b, a divider 530, an arctangent calculation circuit 532, a differentiation circuit 533, and an amplitude adjustment circuit 534.

In the demodulation process, first, the light receiving signal output from the light receiving element 10 is passed through a high pass filter 521 to remove the DC component, and then split into two. One of the split light receiving signals is multiplied by an oscillation frequency signal cosω _M t output from a local oscillator 523a in a multiplier 522a. The other of the split light receiving signals is multiplied by an oscillation frequency signal -sin ω _M t output from a local oscillator 523b in a multiplier 522b. The oscillation frequency signal cos ω _M t and the oscillation frequency signal -sin ω _M t are signals whose phases are shifted by 90° from each other.

The received light signal that has passed through multiplier 522a is passed through low-pass filter 525a and then input to divider 530 as signal x. The received light signal that has passed through multiplier 522b is also passed through low-pass filter 525b and then input to divider 530 as signal y. Divider 530 divides signal y by signal x, and passes signal y/x through arctangent calculation circuit 532 to obtain signal a tan(y/x).

Thereafter, the signal a tan(y/x) is passed through a differentiation circuit 533 and an amplitude adjustment circuit 534 to obtain a sample signal f _d (t), which is a frequency signal.

The circuit configuration of the demodulation circuit 52 has been described above, but the circuit configuration of the above digital circuit is merely an example and is not limited to this. Furthermore, the demodulation circuit 52 is not limited to a digital circuit and may be an analog circuit. The analog circuit may include an F/V (Frequency Voltage) converter circuit and a ΔΣ counter circuit. In the case of frequency demodulation, an analog circuit has the advantage that it is easier to increase the resolution.

The circuit configuration of the demodulation circuit 52 described above is a circuit configuration that outputs frequency information, but it may also be a circuit configuration that outputs phase information. By using this phase information, a displacement meter that determines the displacement of the object to be measured 14 is realized.

1.3 Control Circuit The control circuit 53 performs various calculations on the sample signal f _d (t) output from the demodulation circuit 52. As a result of these calculations, the final output signal of the laser interferometer 1 is obtained.

As mentioned above, the demodulation process is a process for demodulating a sample signal from a received light signal based on light modulated using an optical modulator whose modulation frequency does not change. However, since the vibration elements 3A to 3C vibrate back and forth, the modulation frequency changes. For this reason, if the above demodulation process is performed on a received light signal based on light including the reference light L2 modulated by the optical modulator 12 equipped with the vibration elements 3A to 3C, there is a problem in that an accurate sample signal cannot be extracted.

Therefore, in this embodiment, the demodulation circuit 52 performs intermittent demodulation processing. Specifically, the control circuit 53 performs processing for extracting a signal for a partial period from the sample signal f _d (t) output from the demodulation circuit 52.

The period during which the sample signal f _d (t) is extracted is referred to as the "execution period of demodulation processing." This execution period is set to a period during which the speed of the light reflecting surface undergoing simple vibration can be considered to be almost constant. In such an execution period, the modulation frequency can be considered to be almost constant. Therefore, the control circuit 53 can obtain more accurate Doppler shift information by extracting the signal output during the execution period from the sample signal f _d (t) output from the demodulation circuit 52.

In this specification, the process of extracting a signal for a partial period from the sample signal f _d (t) is described, but the "intermittent demodulation process" also includes a form in which the demodulation circuit 52 performs an operation of intermittently outputting the sample signal f _d (t).

8 is a graph showing the time change of the position L of the light reflecting surface provided on the vibration element 3B vibrating simply, and a graph showing the time change of the modulation frequency _fM by this vibration element 3B. Note that, although the vibration element 3B is given as an example in FIG. 8, the same applies to the other vibration elements 3A and 3C.

The position L of the light reflecting surface shown in Fig. 8 is given by the above-mentioned formula (6). Moreover, the modulation frequency _fM shown in Fig. 8 is given by the above-mentioned formula (8). Therefore, the time change of the position L of the light reflecting surface and the time change of the modulation frequency _fM are both expressed by trigonometric functions. Therefore, the modulation frequency _fM can be considered to be almost constant during the time period T where it reaches its maximum.

Therefore, the control circuit 53 specifies the time t at which the maximum modulation frequency is reached and a time period T of a predetermined width including the time t, based on the modulation frequency _fM . Then, the control circuit 53 extracts the sample signal _fd (t) output from the demodulation circuit 52 during this time period T. By performing such processing, it is possible to obtain information on the Doppler shift with high accuracy. It can also be said that this time period T is the time period during which the instantaneous vibration velocity of the vibration element 3B is the largest.

FIG. 9 is a graph showing the change over time in the Doppler shift _fd output from the demodulation circuit 52 when a laser beam is irradiated onto an object to be measured that is moving at a constant speed.

In the example of Figure 9, the speed of the object being measured is 10 mm/sec. This speed is converted into a Doppler shift of 23.5 kHz. Therefore, in Figure 9, the Doppler shift value of -23,500 Hz is the true value. Figure 9 also shows a region with an error of 1% or less from this true value. If the measurement result falls within this region, it can be said that the accuracy of the measurement result is sufficient.

9 shows the measurement results when the maximum modulation frequency of the vibration element 3B used in the measurement is changed. The maximum modulation frequencies are 0.5 MHz, 1 MHz, 1.5 MHz, 3 MHz, 5 MHz, and 10 MHz from the lowest. Note that the above-mentioned oscillation frequency signal _cosωM and oscillation frequency signal _−sinωM , which are necessary for deriving the measurement results, use the values of the above-mentioned maximum modulation frequencies.

As shown in Figure 9, there is a time period during which measurement results close to the true value can be obtained regardless of the maximum modulation frequency. In addition, the time during which the measurement result is in a region with an error of 1% or less varies depending on the maximum modulation frequency, but a certain amount of time is ensured for each. Therefore, by extracting this time period in the control circuit 53, it can be said that measurements with sufficiently high accuracy are possible.

In addition, the lower the maximum modulation frequency, the longer the time that the measurement results are in the region with an error of 1% or less. However, if the maximum modulation frequency is too low, the accuracy of the measurement results may decrease. Although not shown in Figure 9, a decrease in measurement accuracy was observed when the maximum modulation frequency was set to 0.18 MHz.

Therefore, let us consider the relationship between the wave number corresponding to the time when the modulation signal contained in the reference light L2 modulated by the vibration element is in the region with an error of 1% or less shown in Figure 9, and the maximum modulation frequency of the vibration element. Figure 10 is a table showing the relationship between this wave number and the maximum modulation frequency.

As shown in FIG. 10, when the maximum modulation frequency is 0.18 MHz, the wave number is 2. In this case, as mentioned above, the accuracy of the measurement results was confirmed, and in light of this, it is preferable that the wave number is 3 or more. On the other hand, when the maximum modulation frequency is 0.5 MHz or more, the wave number is 3 or more in all cases. As shown in FIG. 9, when the maximum modulation frequency is in this range, sufficient measurement accuracy is obtained. Therefore, it can be said that it is preferable that the wave number is 3 or more.

As described above, the laser interferometer 1 according to this embodiment includes a light source unit 2, an optical modulator 12, a light receiving element 10, and a demodulation circuit 52. The light source unit 2 emits an output light L1 (first laser light). The optical modulator 12 includes vibration elements 3A to 3C, and modulates the output light L1 using the vibration elements 3A to 3C to generate a reference light L2 (second laser light) including a modulated signal. The light receiving element 10 receives interference light between the reference light L2 and an object light L3 (third laser light) including a sample signal, which is generated when the output light L1 is reflected by the object 14 (target object). The demodulation circuit 52 performs a demodulation process to demodulate the sample signal from the received light signal output by the light receiving element 10. The demodulation circuit 52 performs the demodulation process intermittently.

With this configuration, the demodulation process by the demodulation circuit 52 is performed intermittently, so that, for example, by optimizing the execution period of the intermittent process, more accurate demodulation of the sample signal is possible. For example, by setting the execution period to the time period during which the vibration speed of the vibration elements 3A to 3C is the greatest, the demodulation accuracy of the sample signal can be improved. As a result, it is possible to realize a laser interferometer 1 that can be used for highly accurate displacement measurement and velocity measurement while using the vibration elements 3A to 3C, which are low-cost and simple elements.

The control method for the laser interferometer 1 according to this embodiment is a method for controlling the laser interferometer 1 that includes a light source unit 2, an optical modulator 12, and a light receiving element 10. In this control method, the intermittent demodulation process is performed to demodulate a sample signal from the light receiving signal output by the light receiving element 10.

According to this control method, the demodulation process is performed intermittently, so that, for example, by optimizing the execution period of the intermittent process, more accurate demodulation of the sample signal is possible. For example, by setting the execution period to the time period during which the vibration speed of the vibration elements 3A to 3C is the greatest, the demodulation accuracy of the sample signal can be improved. As a result, it is possible to realize a laser interferometer 1 that can be used for highly accurate displacement measurement and velocity measurement while using the vibration elements 3A to 3C, which are low-cost and simple elements.

The above-described laser interferometer 1 therefore has features such as small size, low cost, high S/N ratio, high accuracy, and wide bandwidth. By using such a laser interferometer 1, it is possible to realize a non-contact vibration measuring device and a vibration control system, a non-contact distance measuring device, a displacement meter, a speed meter, etc., that have these features.

Furthermore, in the demodulation circuit 52 according to this embodiment, it is preferable that the period during which the demodulation process is performed includes the time period T during which the vibration velocity of the vibration elements 3A to 3C is the greatest.

By performing the demodulation process intermittently, including such an execution period, it is possible to reduce demodulation errors in the sample signal. As a result, it is possible to realize a laser interferometer 1 that can be used for highly accurate displacement and velocity measurements while using low-cost, simple elements such as vibration elements 3A to 3C.

In addition, if the execution period is too short, the measurable time may be reduced. In addition, if the number of waves included in the execution period is reduced, the measurement accuracy may be reduced. From this perspective, it is preferable that the execution period of the demodulation process in the demodulation circuit 52 is long enough to include three or more waves of the modulated signal.

By performing the demodulation process intermittently, including such execution periods, it becomes possible to achieve sufficiently high-precision demodulation, for example with an error of 1% or less. As a result, it is possible to realize a laser interferometer 1 that can be used for high-precision displacement and velocity measurements, while using low-cost and simple elements such as vibration elements 3A to 3C.

In particular, it is more preferable that the execution period of the demodulation process is long enough to include 10 or more waves of the modulated signal.

In this case, a comparator or a fast Fourier transform circuit (FFT circuit) can be used in the demodulation circuit 52. This allows the circuit configuration of the demodulation circuit 52 to be simplified. As a result, the laser interferometer 1 can be made more compact.

In addition, the execution period of the demodulation process may be set based on the vibration speed of the vibration elements 3A to 3C, rather than on the wave number. As an example, it is preferable that the execution period of the demodulation process is a time period that includes the time when the vibration elements 3A to 3C reach their maximum vibration speed, and that satisfies that the vibration speed is 90% or more of the maximum vibration speed. In this case, the same effect as above can be obtained.

In addition, taking into account the above findings, the preferred specifications for vibration elements 3A to 3C are as follows:

(a) The maximum modulation frequency is preferably 100 kHz or more.
(b) The period during which the demodulation process is performed includes three or more waves of the modulated signal.

By satisfying these specifications, the following effects can be obtained.
(a) The range of frequencies and velocities that can be measured for the object to be measured 14 can be expanded.
(b) The decrease in measurement accuracy due to a lack of wavenumber can be suppressed.

Furthermore, in order to satisfy the above specifications, as an example, when the vibration frequency f _a is 800 Hz or more, it is preferable to select a vibration element having a displacement amplitude L ₀ of 250 μm or more. On the other hand, when the vibration frequency f _a is less than 800 Hz, it is preferable to select a vibration element having a displacement amplitude L ₀ of 25 μm or more and less than 250 μm.

Therefore, from the viewpoint of easily satisfying the above specifications, it is particularly preferable to use a MEMS vibrating mirror element, a silicon vibrator, or a quartz crystal vibrator for the optical modulator 12.

All or part of the demodulation circuit 52 and the control circuit 53 is composed of hardware having a processor for processing information, a memory for storing programs and data, and an external interface. The processor realizes the functions of the demodulation circuit 52 and the control circuit 53 by reading and executing the various programs and data stored in the memory.

In addition, some or all of the functions of the demodulation circuit 52 and the control circuit 53 may be realized by hardware such as an LSI (Large Scale Integration), an ASIC (Application Specific Integrated Circuit), or an FPGA (Field-Programmable Gate Array).

2. Second Embodiment Next, a laser interferometer according to a second embodiment will be described.
11 to 13 are schematic configuration diagrams each showing an example of a mounting structure of an optical system provided in a laser interferometer according to the second embodiment.

The second embodiment will be described below, focusing on the differences from the first embodiment and omitting the description of similar points. Note that in Figures 11 to 13, the same reference numerals are used for configurations similar to those in the previous embodiment.

The optical system 50A of the laser interferometer 1 shown in FIG. 11 includes a substrate 39. The light source unit 2, the optical modulator 12, and the light receiving element 10 are each mounted on this substrate 39. The light receiving element 10, the light source unit 2, and the optical modulator 12 are arranged on the substrate 39 in this order along a direction perpendicular to the optical path 22 shown in FIG. 11.

The optical system 50A shown in FIG. 11 also includes prisms 40 and 42. The prism 40 is provided on the optical path 24 between the light receiving element 10 and the analyzer 9. The prism 42 is provided on the optical path 20 between the optical modulator 12 and the quarter-wave plate 8.

The optical system 50A shown in FIG. 11 further includes a convex lens 44. The convex lens 44 is provided on the optical path 18 between the light source unit 2 and the polarizing beam splitter 4. By providing the convex lens 44, the emitted light L1 from the light source unit 2 can be focused and effectively utilized.

The optical system 50B of the laser interferometer 1 shown in FIG. 12 is similar to the optical system 50A, except for the arrangement of the elements.

On the substrate 39 shown in FIG. 12, the light source unit 2, the light receiving element 10, and the optical modulator 12 are arranged in this order along a direction perpendicular to the optical path 22 shown in FIG. 12. The prism 40 is provided on the optical path 18, and the prism 42 is provided on the optical path 24.

The optical system 50C of the laser interferometer 1 shown in FIG. 13 has an arrangement in which an optical modulator 12 is incorporated in the optical path connecting the object to be measured 14 and the light receiving element 10.

On the substrate 39 shown in FIG. 13, the light source unit 2, the optical modulator 12, and the light receiving element 10 are arranged in this order along a direction perpendicular to the optical path 22 shown in FIG. 13. The prism 40 is provided on the optical path 18, and the prism 42 is provided on the optical path 24.

The mounting structure shown in Figures 11 to 13 as described above makes it easy to miniaturize the laser interferometer 1. Note that the arrangement of the elements is not limited to the arrangement shown in the figures.

11 to 13, the size of the light receiving element 10 is, for example, 0.1 mm square, the size of the light source unit 2 is, for example, 0.1 mm square, and the size of the optical modulator 12 is, for example, 0.5 to 10 mm square. The size of the board 39 on which these are mounted is, for example, 1 to 10 mm square. This makes it possible to miniaturize the laser interferometer 1 to about the size of this board 39.
In the second embodiment as described above, the same effects as in the first embodiment can be obtained.

The laser interferometer and the control method for the laser interferometer of the present invention have been described above based on the illustrated embodiment, but the laser interferometer of the present invention is not limited to the embodiment, and the configuration of each part can be replaced with any configuration having a similar function. In addition, any other components may be added to the laser interferometer according to the embodiment.

1...laser interferometer, 2...light source unit, 3A...vibration element, 3B...vibration element, 3C...vibration element, 4...polarizing beam splitter, 6...quarter wave plate, 8...quarter wave plate, 9...analyzer, 10...light receiving element, 12...optical modulator, 14...object to be measured, 16...set unit, 18...optical path, 20...optical path, 22...optical path, 24...optical path, 31...element body, 32...light reflecting film, 33...mirror, 34...movable part, 35...light shielding part, 36...vibration piece, 39...substrate, 40...prism, 42...prism, 44...convex lens, 50...optical system, 50A...optical system, 50B...optical system, 50C...optical system, 52...demodulation circuit, 53...control circuit, 351...opening, 521...high-pass filter, 522a...multiplier, 522b...multiplier, 523a...local oscillator, 523b...local oscillator, 525a...low-pass filter, 525b...low-pass filter, 530...divider, 532...arctangent calculation circuit, 533...differential circuit, 534...amplitude adjustment circuit, Ax...rotation axis, L1...emitted light, L2...reference light, L3...object light, T...time period, x...signal, y...signal

Claims (9)

A light source unit that emits a first laser beam;
an optical modulator including a vibration element, the optical modulator modulating the first laser light by using the vibration element to generate a second laser light including a modulation signal;
a light receiving element that receives interference light between a third laser light including a sample signal generated by reflection of the first laser light from an object and the second laser light, and outputs a light receiving signal;
a demodulation circuit that performs a demodulation process to demodulate the sample signal from the received light signal;
Equipped with
The demodulation circuit intermittently performs the demodulation process, and the demodulation process is performed during a period in which the vibration
A laser interferometer characterized by including a time period during which the vibration velocity of an element is greatest . The laser interferometer according to claim 1, wherein the vibration element is an element that generates simple harmonic motion. The laser interferometer of claim 2, wherein the vibration element has a piezoelectric element. The laser interferometer of claim 2, wherein the vibration element has a MEMS vibration mirror element. The laser interferometer of claim 2, wherein the vibration element has a silicon vibrator. The laser interferometer according to claim 2, wherein the vibration element has a quartz crystal oscillator. 7. The laser interferometer according to claim 1, wherein the vibration element has a light reflecting surface that reflects the first laser light, and vibrates in an out-of-plane direction of the light reflecting surface. 8. The laser interferometer according to claim 1, wherein the optical modulator includes a light blocking portion provided on an optical path of the second laser light and configured to block a part of the second laser light. A light source unit that emits a first laser beam;
an optical modulator including a vibration element, the optical modulator modulating the first laser light by using the vibration element to generate a second laser light including a modulation signal;
a light receiving element that receives interference light between a third laser light including a sample signal generated by reflection of the first laser light by an object and the second laser light, and outputs a light receiving signal,
A method for controlling a laser interferometer, comprising intermittently performing a demodulation process for demodulating the sample signal from the received light signal during an execution period that includes a time period during which the vibration velocity of the vibration element is greatest .

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